Use the decisive difference to choose the correct explanation. The final column links to the full discussion.
| Concepts | Decisive difference | Why it matters | Read more |
|---|---|---|---|
| Demand / quantity demanded | Non-price factor shifts D / own-price change moves along D. | Identify the cause before drawing an arrow. | p. 25 |
| PED / YED / XED | Own price / income / another good’s price is the denominator. | Choose the correct formula and interpret the sign. | p. 31 |
| Public good / merit good | Non-rival and non-excludable / underconsumed relative to social optimum. | Free-riding and underconsumption require different explanations. | p. 69 |
| Market / government failure | Market allocation is inefficient / intervention worsens allocation. | A policy cost alone does not establish government failure. | p. 90 |